1. Field of the Invention
The present invention generally relates to a high-frequency heating apparatus, e.g., an electromagnetic induction cooker and a microwave oven. More specifically, the present invention is directed to a high-frequency heating apparatus including a ringing-effect suppressor for a switching element such as an insulated gate bipolar transistor (IGBT), in which an article to be heated such as a metal pan or food is heated by way of an electromagnetic induction effect, or dielectric heating effect.
2. Description of the Related Art
In a conventional inverter type high-frequency heating apparatus such as an electromagnetic cooker and microwave oven, a quasi-E class inverter circuit is built therein, and this inverter circuit is turned ON/OFF in order to produce a desirable high-frequency current. Then, a metal pan or food is heated by way of the electromagnetic induction phenomenon or dielectric phenomenon.
In the conventional quasi-E class inverter circuit, there is employed a bipolar type NPN/PNP transistor functioning as a switching element. Under this circuit condition, the current capacity of this bipolar type switching transistor is inherently not so high, but also the switching speed thereof is not either high. As a consequence, there are problems in such a quasi-E class inverter circuit employing the normal bipolar type switching transistor that the high input power, namely high RF output power is not available. Thus, difficulties exist in the allowable heating power to heat the article to be heated. In other words, the high heating power is not available from the quasi-E class inverter including the normal bipolar type switching transistor.
To overcome the drawbacks, an insulated gate bipolar transistor (referred to as an "IGBT") has newly been utilized as the switching element in the conventional inverter circuit of the high-frequency induction/dielectric heating apparatus, since this insulated gate bipolar transistor has a higher withstand voltage, as compared with the above-described bipolar type PNP/NPN transistor.
FIG. 1 represents an electromagnetic cooker employing the insulated gate bipolar transistor in the DC/AC inverter circuit. This conventional electromagnetic induction cooker is known from, for instance, "AN INDUCTION HEATING SINGLE ENDED PUSH-PULL RESONANT INVERTER USING IGBT" by Yamashita et al, pages 82 to 90, PCIM'88 PROCEEDINGS, 1988.
In the known electromagnetic induction cooker shown in FIG. 1, a DC voltage having a predetermined value derived from a DC power supply circuit 101 is applied to a DC-to-AC inverter circuit 103. Both a heating coil 107 and a resonant capacitor 109 are brought into a series resonant condition by turning ON/OFF an IGBT 113 under the control of a gate driver circuit 115. As a result, the electromagnetic induction effect is caused by the magnetic flux produced from the heating coil 107 so that the eddy current occurs in a metal pan 100 mounted on the heating coil 107. Thus, food stored in this metal pan 100 can be heated.
Referring back to the circuit shown in FIG. 1, a pulse width modulation oscillator 119 (referred to as "a PWM oscillator") modulates a pulse width of an oscillated pulse signal in response to a setting signal derived from an input setting circuit 121. In response to the PWM pulse signal from the PWM oscillator 119, the gate driver circuit 115 turns ON IGBT 113 for a time period determined by the pulse width of the PWM pulse signal.
In FIG. 2, there is shown an equivalent circuit diagram of the insulated gate bipolar transistor 113 employed as the switching element of the DC-to-AC inverter 103. The insulated gate bipolar transistor 113 is constructed of a MOS (Metal Oxide Semiconductor) FET (Field Effect Transistor) 101, a PNP type bipolar transistor TR101 and a resistor R101 connected between the MOSFET 101 and PNP transistor 101. A pulse signal having the voltage that varies from a zero volt to 15 volts and derived from the gate driver circuit 115 is applied to the gate terminal PC of IGBT 113. As represented in FIG. 3A, when the pulse signal having the voltage higher than the threshold voltage "V.sub.TH ", for instance, 3 volts is input to the gate terminal "PG" of IGBT 113, ,the MOS field effect transistor FET101 is, first of all, turned ON which is located at the front stage of this equivalent circuit. Then, when the MOS field effect transistor FET101 is turned ON, the followed PNP transistor TR101 is turned ON. Thus, the collector current "I.sub.C " of IGBT 113 is increased in a straight line form, as illustrated in FIG. 3B.
After a predetermined time period has passed, when the pulse voltage of the pulse signal output from the gate driver circuit 115 is decreased from 15 volts to a zero volt, IGBT 113 is turned OFF. As a result, the voltage between the collector and emitter of the PNP transistor TR101, namely the resonant voltage "V.sub.CE " is obtained as a sinusoidal waveform during the turn-OFF period of IGBT 113, as represented in FIG. 3C.
In case that the electromagnetic cooker shown in FIG. 1 is driven under the source voltage of 200 V at the maximum power of 2KW, the maximum resonant voltage "V.sub.CE " is amount to approximately 1,000 V as shown in FIG. 3C.
As is known in the art, in general, a predetermined capacitance is present between a gate electrode and an emitter of an MOS field effect transistor FET constituting an insulated gate bipolar transistor IGBT. Moreover, an inductance exists in the wiring materials, or wiring patterns between the gate electrode of the MOS field effect transistor FET 101 and the gate driver circuit 115. As a consequence, a so-called "ringing effect" may occur in the gate voltage "V.sub.GE " of IGBT 113 due to the above-described capacitance and inductance. That is to say, even when the pulse voltage of the pulse signal output from the gate driver circuit 115 is reduced from 15 V to a zero V, the gate voltage V.sub.GE of IGBT 113 would not be reduced to a zero volt precisely, and there are produced noise signal components in the gate voltage V.sub.GE around the threshold voltage V.sub.TH. In addition to the above-described problem of the conventional high-frequency heating apparatus, there is another problem. That is, since the gate driver circuit 115 is positioned adjacent to the heating coil 107, the peak voltage caused by the above-explained ringing effect may be increased, or amplified by receiving the magnetic flux produced from the juxtaposed heating coil 107.
When, for instance, the pulse voltage of the pulse signal output from the gate driver circuit 115 is lowered to a zero volt and the gate voltage V.sub.GE of IGBT 113 becomes lower than the threshold voltage V.sub.TH, this IGBT 113 is once turned OFF. However, this gate voltage V.sub.GH exceeds over this threshold voltage V.sub.TH due to the above-described ringing effect and thus the field effect transistor FET 101 shown in FIG. 1 is again turned ON and therefore, a current flows through a diode constructed of the emitter and base electrode of the bipolar transistor TR101.
As previously described, in case that the electromagnetic cooker is operated under the main voltage of 200V and the maximum 2KW input power, the resonant voltage is produced at 200 V in a time period "T.sub.o " during which the ringing effect occurs. As a consequence, the collector current "I.sub.C " of 20A, or near value thereof flows due to the ringing effect and thus a very high power loss may be produced.
Even in case that another conventional electromagnetic cooker is operated under the main voltage of 100V and the maximum input power of 1.2KW, a similar high power loss may occur due to the ringing effect.
Also, in the conventional microwave oven employing the inverter system, namely an insulated gate bipolar transistor as a switching element, a large power loss may occur because of the ringing effect, as previously explained.
Since IGBT 113 is heated due to such a power loss when IGBT 113 is turned OFF, a large heat sink mounted to IGBT 113 is required and also a great cooling fan is necessarily needed.